pg_upgrade fails with missing FTS resources

Hello,

pg_upgrade fails on missing FTS dictionaries requiring external files.
--check fails to detect the incompatibility.

E.g. after installed italian_fts (http://pgxn.org/dist/italian_fts/)
into a database, the pg_upgrade_dump_db.sql contains the command:

CREATE TEXT SEARCH DICTIONARY italian_ispell (
    TEMPLATE = pg_catalog.ispell,
    dictfile = 'italian_ispell', afffile = 'italian_ispell', stopwords
= 'italian_ispell' );

that fails with a:

psql:/var/lib/postgresql/pg_upgrade_dump_db.sql:56891: ERROR:  could
not open dictionary file
"/usr/share/postgresql/9.1/tsearch_data/italian_ispell.dict": No such
file or directory

I think it would be enough to detect the incompatibility at --check
time, as much as it's done for the required libraries, and only for
the builtin dict types.

If you want the issue to be fixed I can try to fix it myself: it seems
easy enough.

-- Daniele